1. Field
The present embodiments relate to a liquid crystal display device, and more particularly to a liquid crystal display device that is adaptive for minimizing flickers and residual images, and a driving method thereof.
2. Related Art
A liquid crystal display device controls the light transmittance of liquid crystal by use of an electric field, thereby displaying a picture. Liquid crystal display devices are mainly classified into vertical electric field applying types and horizontal electric field applying types in accordance with the location of the electric field that drives the liquid crystal.
The vertical electric field applying type drives a liquid crystal of TN mode by the vertical electric field formed between a common electrode and a pixel electrode which are opposite in upper and lower substrates. The vertical electric field applying type liquid has a high aperture ratio, but also has a narrow viewing angle of about 90°.
The horizontal electric field applying type drives a liquid crystal of in-plane switch (hereinafter, referred to as ‘IPS’) mode using a horizontal electric field between a common electrode and a pixel electrode that are disposed to be parallel in a lower substrate. The horizontal electric field applying type has a wide viewing angle of about 160° and a low aperture ratio and transmittance.
In order to improve the low aperture ratio and transmittance of the horizontal electric field applying type, a fringe field switching (hereinafter, referred to as ‘FFS’) type liquid crystal display device which is driven by a fringe field is used. The FFS type liquid crystal display device includes a pixel electrode and a common electrode plate that has an insulating film therebetween in each pixel area, and forms a gap between the common electrode plate and the pixel electrode to be narrower than a gap between the upper and lower substrates, thereby forming the fringe field. Liquid crystal molecules filled in a space between the upper and lower substrates are operated by the fringe field, thereby improving the aperture ratio and the transmittance.
FIG. 1 is a circuit diagram representing one pixel of a FFS type liquid crystal display device according to the related art. FIG. 2 is a cross sectional diagram illustrating a thin film transistor substrate included in the FFS type liquid crystal display device.
Referring to FIG. 1, the FFS type liquid crystal display device includes a plurality of liquid crystal cells Clc which are arranged in a matrix type at the crossing part of data lines DL and gate lines GL. A TFT formed at each of the liquid crystal cells supplies a data signal from the data line DL to the liquid crystal cell Clc in response to a scan signal supplied from the gate line.
In FIG. 2, a thin film transistor substrate of the FFS type liquid crystal display device includes a gate line GL and a data line DL which are formed to cross with a gate insulating film 22 therebetween on a lower substrate 20. A thin film transistor is formed at each crossing part thereof. A common electrode plate 14 and a pixel electrode slit 18 which are formed with the gate insulating film 22 and a passivation film 28 therebetween so as to form a fringe field in a pixel area provided by the crossing structure. A common line 16 is connected to the common electrode plate 14.
The common electrode plate 14 is formed in each pixel area, and receives a reference voltage (hereinafter, referred to as ‘common voltage Vcom’) that drives the liquid crystal through the common line 16 which is formed on the common electrode plate 14 and connected to the common electrode plate 114. The common electrode plate 14 is formed of a transparent conductive layer and the common line 16 together with the gate line 2 is formed of a gate metal layer.
The thin film transistor TFT makes the pixel signal of the data line 4 charged and stored in the pixel electrode slit 18 in response to the gate signal of the gate line GL. For example, the thin film transistor TFT includes a gate electrode 6 connected to the gate line GL. A source electrode is connected to the data line 4. A drain electrode 10 is connected to the pixel electrode slit 18. An active layer overlaps the gate electrode 6 with the gate insulating film 22 to form a channel between the source electrode 8 and the drain electrode 10. An ohmic contact layer 26 for an ohmic contact between the source and drain electrodes 8, 10 and the active layer 24. A semiconductor pattern 30 includes the active layer 24 and the ohmic contact layer 26.
The pixel electrode slit 18 is connected to the drain electrode 10 of the thin film transistor TFT through a contact hole 12 which penetrates the passivation film 28 that overlaps the common electrode plate 14. The pixel electrode slit 18 forms a fringe field with a common electrode plate 14 to make liquid crystal molecules rotate by dielectric anisotropy. The liquid crystal molecules are arranged in a horizontal direction between a thin film transistor substrate and a color filter substrate. The transmittance of the light transmitted through a pixel area is changed in accordance with the degree of rotation of the liquid crystal molecules, thereby realizing a gray level.
A storage capacitor Cst that stably maintains the video signal supplied to the pixel electrode slit 18 is formed between the common electrode plate 14 and the pixel electrode slit 18. The storage capacitor Cst stores the voltage of the liquid crystal cell Clc at a fixed level.
The liquid crystal display device is driven by an inversion method of periodically inverting the polarity of the data charged in the liquid crystal cell to reduce flickers and residual images. The inversion method is classified into a line inversion method where the polarity of data is inverted between the adjacent liquid crystal cells in a vertical line direction. A column inversion method has the polarity of data inverted between the adjacent liquid crystal cells in a horizontal line direction. A dot inversion method has the polarity of the data inverted between the adjacent liquid crystal cells in the vertical line direction and the horizontal line direction.
In the dot inversion method, as shown in FIG. 1, the polarities of the data supplied to each of the adjacent liquid crystal cells are contrary to each other in the vertical direction and the polarities of the data supplied to each of the adjacent liquid crystal cells are contrary to each other in the horizontal direction. The polarity of the data is inverted for each frame (Fn−1, Fn).
A feed-through voltage ΔVp is generated that results in the flickers and the residual images during the driving of the liquid crystal display device by the dot inversion method.
Referring to FIG. 4, a gate voltage Vg is supplied to the gate electrode 8 of the TFT 6 and a data voltage Vd is supplied to the source electrode 10. If a gate high voltage Vgh being not less than a threshold voltage of the TFT 6 is applied to the gate electrode 8 of the TFT 6, a channel is formed between the source electrode 10 and the drain electrode 12, and the data voltage Vd is charged in the liquid crystal cell Clc and the storage capacitor Cst through the source electrode 10 and the drain electrode 12 of the TFT.
The feed-through voltage ΔVp is the difference between the data voltage and the voltage charged in the liquid crystal.
The feed-through voltage ΔVp is not fixed because the polarity of the data is inverted for each frame (Fn−1, Fn) or in accordance with the gray level. Thus, the common voltage Vcom is not located in the center of the positive data voltage and the negative data voltage. For example, the feed-through voltage ΔVp in the positive data voltage for displaying a white color and the feed-through voltage ΔVp in the negative data voltage for displaying a white color are not the same in magnitude, thus an effective value of the data voltage for expressing the same gray level is not fixed in accordance with the polarity. Accordingly, the common voltage being a DC voltage cannot be set as an optimal common voltage value corresponding to the center of the positive data voltage and the negative data voltage. A brightness difference is generated between frames, thereby still resulting in the flickers and the residual images.
Accordingly, a liquid crystal display device that is adaptive for minimizing flickers and residual images is desired.